Jesse Jackson, whose candidacy is now official, will not become president of the United States, or even gain the Democratic nomination. I understand that.

He will be seen by many Americans as merely a "black candidate." I understand that, too. I even understand why a significant number of black leaders have chosen to endorse someone else. It is, as they say, a free country.

What I don't understand is the attitude on the part of so many black civil rights leaders and politicians that Jesse Jackson shouldn't run; not merely that his candidacy is futile but that it is wrong, inimical to the interests of blacks.

Detroit Mayor Coleman Young was on the evening news a couple of days ago saying that Jackson's anticipated candidacy was a trap. Mike Wallace, in the "60 Minutes" interview in which Jackson announced his intention to announce, ticked off an impressive list of well-known blacks who say "no to Jackson": Andrew Young, Coretta Scott King, Tom Bradley, Julian Bond, Wilson Goode, Joe Lowery, John Jacob.

Ben Hooks, executive director of the NAACP, has been on record for months as opposing "a black candidacy," not because he doubts the right of a black to seek the nation's highest office, he insists, but because a black candidate would make Reagan's reelection easier.

But while that argument makes sense in terms of a Jackson challenge as an independent in the general election, what possible sense can it make in the Democratic primaries? That's the part I don't understand. The prevailing assumption is that a Jackson bid would hurt Walter Mondale and help John Glenn, at least in the South, where, absent the black vote, those two are running even. But even supposing that Jackson would drain enough votes from Mondale to hand the Democratic nomination to Glenn, how does that help Reagan?

Are there pro-Mondale Democrats whose second choice would be Reagan rather than Glenn (or Askew or Cranston or Hart or Hollings or McGovern)? Are there Republicans and independents who, given the chance, would vote for Mondale but who, absent Mondale, would turn to Reagan?

Ah, but what of all those previously unregistered black voters who would sign up for a Jackson candidacy but go back to sleep if Jackson failed to get the nomination? I expect there are a lot of people in that category. But these are votes which, by definition, wouldn't be available to Mondale except as the result of a Jackson run.

Jackson, these respected leaders will tell you, threatens to split the Democratic Party at a time when unity is of the essence. Two questions: 1) Is there ever a time when the party faithful don't consider unity of the essence? 2) If unity is such a big deal, why were there no screams of party-splitting when McGovern joined the fray, or when an Askew, who makes less of a dent in the polls than Jackson, refuses to bow out?

So what is behind all this sly "Of course a black has a right to run, but not in 1984" talk? Do you suppose that a recent ABC News poll sheds any light, perhaps a green- tinged light, on that question?

This was a special poll of black, voting-age Americans, and one of the questions was: who do you think is the most important black leader in America? Fifty-one percent named Jesse Jackson. The next highest scores were 8 percent for Andrew Young, 4 percent for Martin Luther King Sr., 2 percent for Benjamin Hooks, and 1 percent each for Julian Bond, Coretta Scott King, Harold Washington and Coleman Young.